1. Field of the Invention
The present invention relates to a quartz crystal oscillator for surface mounting, and more specifically to a crystal oscillator having an inspection terminal, which is used for inspecting the characteristics of a quartz crystal blank, on the outer surface of the container body.
2. Description of the Related Arts
A surface-mount type quartz crystal oscillator wherein a quartz crystal blank and an IC (integrated circuit) chip equipped with an oscillation circuit using the crystal blank are accommodated in a container body for surface mounting is widely built in portable electronic devices represented by mobile phones as a reference source for frequency and time because of small size and light weight thereof. Examples of such surface-mount type crystal oscillators include a crystal oscillator wherein a container body having a recess formed on one of principal surfaces is used, and an IC chip and a crystal blank are encapsulated in the recess; and a crystal oscillator wherein a container body having recesses formed on both of the principal surfaces is used, and a crystal blank is encapsulated in one of the recesses and an IC chip is accommodated in the other recess.
FIG. 1A is a sectional view showing an example of the configuration of a conventional surface-mount type crystal oscillator; and FIG. 1B is a schematic diagram showing the disposition and wire connection of each circuit and each terminal in the crystal oscillator shown in FIG. 1A.
The illustrated crystal oscillator uses container body 1 having a recess; wherein IC chip 2 and crystal blank 3 are accommodated in the recess, and container body 1 is covered with metal cover 4 to close the recess, and thereby IC chip 2 and crystal blank 3 are hermetically encapsulated in container body 1.
Container body 1 has a flat and substantially rectangular parallelepiped shape, and in one of the principal surfaces, a substantially rectangular recess for accommodating IC chip 2 and crystal blank 3 is formed. Such container body 1 is composed of laminated ceramics having a substantially rectangular flat bottom-wall layer, and a frame-wall layer having an opening in the center portion provided on the bottom-wall layer. The inner side wall of the recess is defined by the frame-wall layer, and the upper surface of the bottom-wall layer exposed by the opening of the frame-wall layer becomes the inner bottom surface of the recess. A step portion is formed on the inner wall of the recess, and a pair of holding terminals 5 for holding crystal blank 3 is provided on the upper surface of the step portion. A plurality of circuit terminals used for electrical connection to IC chip 2 are provided on the inner bottom surface of the recess. Mounting terminals 6 used when the crystal oscillator is surface-mounted on a wiring board are provided on the four corner portions of the outer bottom surface of container body 1. Mounting terminals 6 include: a power supply terminal (Vcc); an output terminal (OUT); a grounding terminal (GND); and an AFC (automatic frequency control) terminal to which an AFC signal is supplied. Mounting terminals 6 are electrically connected to corresponding circuit terminals via conducting paths formed in container body 1. Inspection terminals 6A(X1), 6A(X2) are provided on a pair of opposite outer side surfaces of container body 1, respectively. As described below, inspection terminals 6A are electrically connected directly to crystal blank 3, and are used for inspecting the characteristics of crystal blank 3 as a quartz crystal element. All of the circuit terminals, holding terminals 5, mounting terminals 6 and inspection terminals 6A are provided as electrode layers formed on the surface of the laminated ceramics.
A pair of holding terminals 5 are electrically connected to a pair of corresponding terminals in the circuit terminals via conducting paths formed in container body 1, and also electrically connected to a pair of inspection terminals 6A.
IC chip 2 is formed by integrating electronic circuits that constitute oscillation circuit 15 using crystal blank 3. Such a crystal oscillator is constituted as a voltage-controlled crystal oscillator (VCXO), a temperature compensated crystal oscillator (TCXO), a clock oscillator or the like, functional circuits corresponding to these uses are also integrated in IC chip 2. In IC chip 2, since electronic circuits are formed on one of principal surfaces of a semiconductor substrate by an ordinary semiconductor device fabricating process, the principal surface of the semiconductor substrate, on which these electronic circuits are formed, will be referred to as the “circuit forming surface” of the IC chip. On the circuit forming surface, a plurality of IC terminals 7 for connecting IC chip 2 to exterior circuits are provided corresponding to the locations of circuit terminals in container body 1. IC terminals 7 include: a power supply terminal (Vcc); an output terminal (OUT); a grounding terminal (GND); and an AFC terminal; and also include a pair of crystal IC terminals (X1, X2) used for electrical connection with crystal blank 3. IC chip 2 is fixed in the recess of container body 1 so that the circuit forming surface faces the inner bottom surface of the recess using a flip-chip bonding technique. Specifically, IC chip 2 is fixed in the inner bottom surface of the recess by electrically and mechanically connecting IC terminals 7 to the circuit terminals by ultrasonic thermal compression bonding using bumps.
By thus fixing IC chip 2 in the container body 1, crystal IC terminals 7(X1), 7(X2) are electrically connected to the holding terminals 5 via circuit terminals and conducting paths; and the power supply terminal (Vcc), the output terminal (OUT), the grounding terminal (GND), and the AFC terminal among IC terminals 7 are electrically connected to the corresponding ones among mounting terminals 6, respectively.
When the crystal oscillator has a stand-by function, in mounting terminals 6 and IC terminals 7, a stand-by terminal is provided in place of the AFC terminal.
When the crystal oscillator is constituted as a temperature compensated crystal oscillator, a read/write terminal (communication terminal) for reading and writing data from and to a temperature compensating mechanism in IC chip 2 is provided, for example, on the outer side surface of container body 1, and a circuit terminal electrically connected to the read/write terminal is also provided. An IC terminal connected to the temperature compensating mechanism is provided also in IC chip 2, so that temperature compensating data can be read from and written in the temperature compensating mechanism from the read/write terminal via the circuit terminal.
As shown in FIG. 2, crystal blank 3 is, for example, a substantially rectangular AT-cut quartz crystal blank, and excitation electrodes 9a are formed on the both principal surfaces, respectively. Lead-out electrodes 9b are extended from excitation electrodes 9a toward the both sides of one end portion of crystal blank 3, respectively. Each of lead-out electrodes 9b is formed so as to turn back between the both principal surfaces of crystal blank 3 at the location on the end edge portion of crystal blank 3. Crystal blank 3 is held in the recess of container body 1 at the location where a pair of lead-out electrodes 9b are drawn out by fixing these lead-out electrodes 9b to holding electrode 5 with electrically conducting adhesive 10, and electrically thus connected to IC chip 2. At this time, crystal blank 3 is directly electrically connected to inspection terminals 6A(X1), 6(X2).
Such a crystal oscillator can be assembled by fixing IC chip 2 on the inner bottom surface of the recess in container body 1, fixing crystal blank 3 on the step portion of the recess, and thereafter bonding metal cover 4 to the open end of the recess to close the recess by, for example, seam welding. Next, the vibration characteristics, such as crystal impedance (C1), of crystal blank 3 as crystal unit 3A are inspected using inspection terminals 6A(X1), 6A(X2). As a result of the inspection, crystal oscillators having favorable vibration characteristics and satisfying standards for a crystal unit are made to be acceptable products.
When metal cover 4 is bonded to container body 1 using seam welding or the like, stress due to difference in the coefficients of thermal expansion between metal cover 4 and container body 1 is generated, strain occurs in container body 1 by the stress, electrically conducting adhesive 10 for fixing crystal blank 3 is also affected by the stress, and the state of the vibration system including crystal blank 3 is also changed. The vibration characteristics of crystal blank 3 may be then changed in the states before and after bonding metal cover 4. Therefore, when requirements of stability and reliability of a crystal oscillator are high, the inspection for the characteristics of crystal blank 3 after metal cover 4 has been bonded to container body 1 to encapsulate IC chip 2 and crystal blank 3 in container body 1 becomes essential. In addition, since vibration characteristics such as C1 change due to the state of fixing crystal blank 3 by electrically conducting adhesive 10, inspection terminals 6A(X1), 6A(X2) can also be used for measuring the vibration characteristics before encapsulated by metal cover 4.
However, since inspection terminals 6A(X1), 6A(X2) used for inspecting the characteristics of crystal blank 3 as crystal unit 3A are exposed to the outer side surface of container body 1, the surface-mount type crystal oscillator having the above configuration is prone to be affected by other electronic devices and wiring patterns on the wiring board when the crystal oscillator is mounted on the wiring board. In particular, since inspection terminals 6A(X1), 6A(X2) are always electrically connected to crystal blank 3, the stray capacitance of the circuit containing crystal blank 3 is increased, and the deterioration of oscillation characteristics, such as the variation of oscillation frequencies, is caused.
Inspection terminals 6A(X1), 6A(X2) are electrically connected not only to crystal blank 3 but also to IC chip 2. Therefore, when the vibration characteristics of crystal blank 3 is measured, inspection terminals 6A(X1), 6A(X2) are subjected to the electrical effects of IC chip 2 wherein oscillation circuit 15 is integrated, and the vibration characteristics of crystal blank 3 alone cannot be strictly measured.
Furthermore, in crystal unit 3A constituted by hermetically encapsulating crystal blank 3 in a container, so-called DLD (drive level dependency) characteristics deteriorate due to, for example, dust adhered on excitation electrode 9a or lead-out electrode 9b. Therefore, it is a general practice to make crystal blank 3 strongly excited via inspection terminals 6A(X1), 6A(X2) to shake dust adhered on excitation electrode 9a or lead-out electrode 9b down. In this case, since inspection terminals 6A(X1), 6A(X2) are also electrically connected to IC chip 2, there is a problem that electronic circuits in IC chip 2 are damaged by voltage used for strongly exciting crystal blank 3.
As a method for solving such a problem, a method wherein the conducting path that connects inspection terminals to the IC chip is drawn out to the outer side surface of the container body and disconnected, and after measuring the vibration characteristics of the crystal blank, the disconnected conducting path is connected by electrically conducting adhesive or the like to complete the crystal oscillator, is proposed in, for example, Japanese Patent Application Laid-Open No. 2006-279979 (JP-2006-279979A). However, this method has a problem that the crystal oscillator tends to cause defective appearance wherein the electrically conducting adhesive mounded from the outer side surface thereof, and the manufacturing process is complicated to lower productivity.
Similarly, Japanese Patent Application Laid-Open No. 9-162643 (JP-9-162643A) discloses an oscillator module including a pair of external terminals connected to a crystal blank and a plurality of external terminals connected to an oscillation circuit, which can independently measure the characteristics of the crystal blank and the characteristics of the oscillation circuit, and can be constituted as a crystal oscillator by electrical connection between the external terminals.